Zoo setting up billions to feed ape? (5)

'zoo' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'up billions to feed ape?' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'billions' becomes 'b' (abbreviation).
'to' becomes 'bu' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'feed' means one lot of letters goes inside another (one word is fed into another).
'ape?' becomes 'do' (both can mean to mimic).
'b'+'bu'='bbu'
'bbu' backwards is 'ubb'.
'ubb' placed into 'do' is 'DUBBO'.

'setting' acts as a link.

Can you help me to learn more?